1. Field of the Invention
The present invention generally relates to a control method of cell formation and a mobile communications system therewith, and specifically relates to the cell formation method and the mobile communications system therewith, wherein one or more mobile stations are served by a base station that is capable of changing and securing a service area of the mobile stations by changing the cell formation while communications are going on.
The present invention further relates to the base station and mobile station that can communicate according to the cell formation control method.
2. Description of the Related Art
An area where a base station can provide a communication service, i.e., a cell, depends on the quality of a control signal received by a mobile station, the control signal being for connecting the mobile station with the base station. When the base station serves a cell, a conventional method has been that transmission electric power of the control signal is predetermined, and the predetermined value is used. In recent years, an autonomous cell formation control method has been proposed, wherein a base station checks cell-formation of adjacent cells, and adjusts its cell formation such that areas not covered by the adjacent cells are efficiently covered. The autonomous cell formation control method mediates traffic congestion by controlling the cell formation according to a congestion state of the adjacent cells, and can raise frequency use efficiency. If the technology of such autonomous cell formation control is applied to a mobile communications system, a base station will be capable of changing cell formation, i.e., increasing and decreasing the radius of the cell, while communications are continuing.
When the cell radius is expanded, an increased number of base stations will become available to a mobile station that is in communication with a first base station. The communication connection to the first base station can be switched to a second base station, if the second base station is carrying a lower amount of traffic, such that a higher through put may be obtained.
On the other hand, when the cell radius of the first base station with which the mobile station is connected is reduced, it is necessary for the mobile station to switch the ongoing communication connection to another base station before the service of the first base station to the mobile station becomes unavailable.
However, according to the autonomous cell formation control of a conventional base station, a relief is not provided to the mobile station that would be made outside the service area of the base station before the base station actually reduces its service area.
For example, as shown in FIG. 16, when a base station 3 reduces its service area from an area indicated by a dotted line to an area indicated by a solid line, although a mobile station 3 served by the base station is thereby put outside the reduced service area of the base station 3, the mobile station 3 can be switched to the base station 2 by hand-over, such that the communication can continue. However, when a base station 1 reduces its service area from an area indicated by a dotted line to an area indicated by a solid line, there is no adjacent base station to perform hand-over for a mobile station 1 that is being served by the base station 1. Consequently, when the base station 1 reduces its service area, the mobile station 1 will be put outside the reduced service area, and the communication will be disconnected.
Thus, the conventional problem is that a normal communication of a mobile station with the first base station cannot be continued due to disconnection, a packet loss and the like, when the first base station reduces its cell radius, i.e., service area.
Even if there is the second base station available for the service to be continued in an adjacent area, hand-over has to be performed from the first base station to the second base station. Further, there is a possibility that the second base station may later reduce its service area. In this case, further hand-over is required from the second base station to a third base station, or back to the first base station, as the case may be.